The present invention relates to an air-fuel ratio control apparatus for internal combustion engines which uses catalyst for purifying exhaust gases at the upstream and downstream sides of the exhaust pipe of the internal combustion engine, to thereby control the air-fuel mixture ratio by regulating the amount of fuel injection to cylinders according to engine operating conditions.
As a conventional technology for warming up an exhaust gas purifying catalyst in a short period of time, injection dither is proposed to increase and decrease the amount of fuel injection for alternately leaning and enriching fuel in relation to the stoichiometric air-fuel ratio (.lambda.=1). The injection dither is performed to supply oxygen (O.sub.2) and unburned fuel (HC and CO) to the catalyst by increasing or decreasing the amount of fuel to be injected, for reaction and heating in the presence of the catalyst, thereby raising the catalyst temperature to accomplish early activation of the catalyst. Generally, when the injection dither is carried out, the target air-fuel ratio is changed over by leaning and enriching the fuel at intervals of 1 to 5 injections by fuel injectors.
However, the method of effecting catalyst activation by the injection dither is effective only for warming up one catalyst from a cold state. However, in a system in which catalysts are arranged in series in the exhaust pipe, the catalyst on the upstream side is early warmed up. If the injection dither is kept operated after the completion of the warm-up of the catalyst, the catalyst on the upstream side will be overheated, exceeding the permissible temperature. If, however, the injection dither is stopped upon the completion of the catalyst warm-up in an attempt to obviate this disadvantage, there will take place such a problem that the catalyst on the downstream side will not lower to a desired temperature.
In JP-A-8-158858, a front catalyst is mounted on the upstream side of the exhaust pipe, while a main catalyst is disposed on the downstream side. During the period of front catalyst activation after engine start-up, the exhaust gas temperature rises to positively activate the front and main catalysts. After the activation of the front catalyst, the exhaust gas temperature rise stops and the main catalyst is activated by the use of the injection dither which increases and decreases the air-fuel ratio. The front catalyst can be prevented from overheating.
In this apparatus, however, the injection dither is carried out after the activation of the front catalyst. The heating of the front catalyst can not be prevented, presenting a problem of catalyst overheat. Also if one tries to restrain the heating of the front catalyst, there will occur such a problem as a deteriorated activation effect of the main catalyst.